This invention relates to the preparation of heteropoly acids of tungsten and molybdenum, and more specifically relates to a method for producing such heterpoly acids from alkali metal salt solutions using cation exchange membranes.
The classical method of making what is generally known as "alkali free" 12-tungstophosphoric acid is by ether extraction of an acidified mixture of sodium tungstate and sodium hydrogen phosphate. The tungstophosphoric acid ether complex may be decomposed by evaporation to crystals. This method is described in, Inorganic Synthesis, 1, pages 132-133 (1939), H. S. Booth, Ed., McGraw Hill Book Company, New York, New York. Alternatively the ether complex may be decomposed by heating with water or dilute hydrochloric acid solution, as described in ibid, 1, Page 133; J. Chem. Soc., pages 3189-3193 (August 1962); and J. Applied Chem. (London), 2, pages 262-264 (1952). The principle disadvantages of this method are the use of ether for extraction, which makes a large manufacturing process based upon it hazardous, and also the fact that yields are relatively low and the products relatively unstable. Such instability is primarily due to the presence of residual alkali metal. Various attempts have been made to remove such residual impurity, as exemplified by the use of various cation exchange resins in contact with dilute solutions of the product. See U.S. Pat. No. 2,503,991.
Another method for the preparation of heterpoly acids is described in U.S. Pat. No. 3,446,557, assigned to the assignee of the present application, in which a hydrolyzable alkali-free source of tungsten, for example a tungsten hexahalide, is hydrolyzed in an aqueous medium including an acid of the hetero atom. This technique generally requires the removal of residual mineral acid anions which lead to instability of the heteropoly acid product. A technique for the removal of mineral acid anions from the heteropoly acid product using anion exchange resins is disclosed in U.S. Pat. No. 3,361,518, assigned to the present assignee. However such removal is time consuming and expensive and the anion resin tends to hold the heteropoly acid as well as the mineral acid ions causing very low yields.
A further method for the preparation of heteropoly acids is described in U.S. Pat. No. 2,503,991, wherein an aqueous solution containing a mixture of an alkali metal salt of the desired hetero atom with an alkali metal molybdate or tungstate is contacted with a cation exchange resin thereby to remove the alkali and convert the reactive anions to the desired heteropoly acid. This technique becomes impractical in the preparation of heteropoly acids in which the hetero ion is insoluble. In the preparation of silicotungstic acid or silicomolybdic acid for instance, sodium silicate converts to silica gel when the sodium is removed and physically plugs the resin bed or column.
Another method is disclosed in U.S. Pat. No. 3,288,562 assigned to the present assignee which involves forming an aqueous solution of an alkali metal tungstate or molybdate, adjusting the pH with phosphoric acid, and contacting this solution with a cation exchange resin. Because this technique relies upon the use of cation exchange resins it is applicable only to the preparation of acids having a soluble hetero ion.
The search continues for a preparation technique for heteropoly acids of tungsten and molybdenum which has a relatively high efficiency, is broadly applicable regardless of the solubility of the hetero ion, and results in a product substantially uncontaminated with either alkali metal cations or mineral acid anions which would tend to degrade the hetero acid product.